It is well-known in the arts to frequency-modulate oscillators. Switched DC-DC converters are also well known, in which for example transistors are switched with a switching frequency, wherein the switching frequency is generated by an oscillator. Especially when the switching frequency is high, this leads to noise and undesired electromagnetic interferences (EMI).
For a variety of reasons high-frequency conversion systems generate more noise than their low-frequency counterparts.
More recently it became therefore necessary to frequency-modulate oscillators used in switched DC-DC converters. In this case a so-called spread-spectrum function is used to keep noise levels of the converter down. The DC-DC converter is not switched anymore with a fixed frequency, but the switching frequency is varied around a center frequency to get a frequency spectrum which is spread, thus the energy is not concentrated to only one frequency.
One known parameter of frequency modulation is the modulation index or modulation ratio which is defined as the ratio between the maximal difference or deviation of the modulated frequency to the center frequency. The modulation index thus indicates how much the modulated frequency varies around the center frequency. The modulation index can be expressed by formula (1):
                              M          I                =                              Δ            ⁢                                                  ⁢            F                                F            c                                              (        1        )            wherein MI is the modulation index with 0≦MI≦1ΔF is the maximal difference between the modulated frequency and the center frequencyFc is the center frequency.
For a given frequency modulation it is important that the modulation index is kept constant. In conversion systems which employ fixed-frequency oscillators, a spread spectrum can easily be accomplished by using a linear ramp generator with a specific magnitude as a modulating signal to get the desired modulation index. To keep the modulation index constant the maximum amplitude of the linear ramp generator is kept constant by means of current or voltage control.
In HF conversion systems, hysteretic/ring-oscillating regulation methods are often employed.
DC-DC converters or oscillators employing hysteretic/ring oscillating methods have a frequency which is inherently variable and therefore difficult to predict. There are methods to lock the operating frequency of these converters to a given frequency. However, the frequency gain of the resulting system is highly variable, non-linear and dependent on external conditions. It is for example dependent on the voltage levels at the input and at the output and it depends on the equivilant series inductance of the output capacitor in a DC-DC converter (ESL). These oscillators have a non-linear control behavior.
Thus, the magnitude of the modulation signal required to achieve a given modulation index is highly variable and keeping the modulating current to a specified value does not necessarily keep the modulation index constant.